During the Christmas season of 1938, the Doctor finds himself on a damaged alien spacecraft in Earth's orbit. He escapes the exploding ship and then falls to Earth by rapidly donning an impact space suit, though in his haste, the helmet is put on backwards. On crashing to Earth, he is found by Madge Arwell (Claire Skinner), wife of Reg (Alexander Amstrong) and mother of two children, Lily (Holly Earl) and Cyril (Maurice Cole). She helps the Doctor to his TARDIS, and the Doctor promises to repay her for her kindness. Three years later, during World War II, Reg is reported missing in action when the Lancaster bomber he was piloting disappeared over the English Channel. Madge is told this via telegram just before Christmas, but decides not to tell her children yet, hoping to keep their spirits up through the holiday. Madge and the children evacuate London to a relative's house in Dorset, where they are greeted by the Doctor, calling himself "the Caretaker"; Madge does not recognise him from their previous encounter, as his face had been hidden by the backwards helmet.
The Doctor has prepared the house specially for the children and the holiday; though the children are pleased, Madge privately explains about Reg's death to the Doctor and insists he not overindulge the children. During the first night, Cyril is lured into opening a large glowing present under the Christmas tree, revealing a time portal to a snow-covered forest. The Doctor shortly discovers Cyril's absence and follows him with Lily; the two eventually track Cyril down to a strange lighthouse-like structure. Madge, finding her children missing, soon follows them into the forest, but is met by three miners in space suits from the planet Androzani Major (Bill Bailey, Paul Bazely, and Arabella Weir). She holds them at gunpoint and is taken back to their excavation walker and told that the forest of the planet they are on is scheduled to be melted by acid rain within minutes, killing anything within it. At the lighthouse, Cyril is met by a humanoid creature made of wood; it places a simple band of metal around his head like a crown. Lily and the Doctor arrive, followed by another wood creature, but find the creatures have rejected Cyril as he is "weak", as they do the Doctor. The Doctor concludes that the life forces of the trees in the forest are trying to escape through a living creature, the crown acting as an interface.
The miners are teleported away safely before the rain starts after helping Madge to locate where her children are. Madge, using the little knowledge she knows of flying a plane from Reg, directs the walker to the lighthouse and safely reunites with her children as the acid rain starts. The wood creatures identify her as "strong", and the Doctor realises they consider her the "mothership", able to carry the life force safely. Donning the band, Madge absorbs the life force of the forest, allowing her to direct the top of the lighthouse as an escape pod away from the acid rain and into the time vortex. To get them home, the Doctor directs her to think of memories of home, allowing Madge to revisit her fond memories of Reg, shown on screens within the pod. The Doctor urges her to continue to show even Reg's death, revealing to Lily and Cyril what happened to their father. Soon, the escape pod safely leaves the time vortex, landing just outside the house in Dorset, and the life force of the forest have converted themselves to ethereal beings within the time vortex. The Doctor steps outside while Madge starts to explain Reg's death to Lily and Cyril, but he returns to interrupt her and to tell her to come outside. There stands Reg and his Lancaster; he had followed the bright light of the escape pod into the time vortex and came out safely along with the pod at Dorset. The family has a tearful reunion as the Doctor watches.
As Madge and her family turns to celebrate Christmas, the Doctor attempts to slip away, but Madge catches him, and as she sees the TARDIS realises that he is the man in the space suit from three years back. She insists on him staying for Christmas dinner, but when the Doctor reveals he has other friends out there that believe he is dead, Madge convinces him to go to see them at Christmas. The Doctor offers Madge his help if she ever needs it again. Later, the Doctor arrives outside the home of Amy (Karen Gillan) and Rory (Arthur Darvill), two years since he left them there ("The Wedding of River Song"). They invite him to have Christmas dinner, as Amy had developed a habit of setting him a place at the dinner table, and the Doctor finds himself experiencing the human emotion of tears of joy.
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